The Ground Fault Circuit Interrupter has undergone steady improvement in regard to achieving increased reliability and lower cost since its introduction as a major factor in the field of electrical safety. Only the continuing need for its manual resetting remains as a substantial deterrent to universal acceptance.
This invention relates to improvement of Ground Fault Circuit Interrupter function by safely eliminating this need for manual resetting, which is particularly objectionable wherein tripping of the GFCI may be due to brief transient faults or those line and load characteristics which are capable of tripping the GFCI device, but which do not constitute true faults. Where load circuits may be infrequently attended by personnel, the problems created by the need for manual resetting after unwarranted tripping become extreme and GFCI protection cannot be afforded due to the possibility of prolonged periods in which critically needed devices such as freezers, pumps, etc., would be unnecessarily put out of service. Manual resetting has been required in the interest of consumer safety to give personnel the opportunity to assess a possible fault situation before attempting reset which causes at least a momentary re-energizing of the load circuit.
The automatic reset feature in accordance with the herein disclosed invention accomplishes a determination of hazardous fault conduction level without repeating the original fault current flow which may have been very large, far beyond the tripping level required for the GFCI, and limited only in duration by the tripping time of the GFCI. The automatic reset circuit not only eliminates unwarranted down time, but employs a safer procedure than that involving the manually operated reset switch which requires that the power circuit be experimentally re-energized to confirm or deny the presence of a less than obvious fault. The fault current sensitivity of the automatic reset circuit may be made slightly more or considerably more than that of the GFCI it controls in accordance with the level of concern for safety in individual situations. A power circuit tripped by a fault of slightly more than 5 ma, for example, may thus be kept off in the presence of a continuing fault with an impedance that would permit a current flow of 4 or 3 or even as little as 2 ma under energized load circuit conditions. With automatic reset capability, standards for sensitivity to initial tripping in the GFCI circuit may also be increased to 4, or even 3 ma, for additional protection against the hazard of ground faults without increases in the losses and inconvenience of unwarranted down time since power circuits would be automatically and immediately restored if unwarranted tripping should occur. Test switches may be located in positions remote from the GFCI, temporarily connecting a resistance from the line to a grounding lead in a tool or appliance, to provide convenience and motivation for more frequent testing and optional on-off switching by UFCI control.
The Ground Fault Circuit Interrupter as referred to herein is the conventional device designed to supply A.C. power from a line source to a load circuit, and to interrupt said power in response to detected ground faults in the load circuit which create a difference between current level in the hot conductor and the return, or neutral conductor, due to passage of current from the load circuit to ground by a path other than that provided by the return, or grounded neutral conductor. Such current differences are made evident for detection by the generation of current in a differential current transformer surrounding both conductors (also variously called a core balance current transformer and a zero sequence current transformer). Only one such transformer is shown to simplify GFCI representation, although a second is commonly employed to detect neutral to ground faults.
The Ground Fault Circuit Interrupter as referred to herein more specifically refers to the electrically resettable type of the above described GFCI which is equipped with a normally closed reset switch through which power is supplied to its control module and wherein reset is accomplished in the absence of a hazardous fault by first opening the switch to interrupt power to the module and then closing the switch to restore power to the module. Interruption and restoration of power to the entire GFCI unit also functions to accomplish reset in this type of unit as differentiated from the type in which the reset button is a physical extension of a solenoid tripped switch. Tripping current is supplied by the GFCI circuit, but manual contact closure is required for resetting. The automatic reset circuit consists of a controlled reset switch connected in place of, or in series with, the standard reset switch, or between the power source and the GFCI, a current limited ground-fault sensing device, a coupling means between the controllable switch and the fault sensing device which may be variously an emission path, a conductive connection, a magnetic path, or other means of physical influence, and a means of assuring an initial opening of the controllable switch. While it is recognized that the current limited ground-fault sensing device when integrated into the GFCI circuitry may be coupled in a manner that affects a reset without simple and obvious connection to reset switch terminals, it is felt that the conventional reset procedure requiring initial break and subsequent make now made available at the switch terminals is a valuable one, affording certain fail-safe advantages and should be retained.
After tripping of the GFCI, the reset circuit is first opened by a fault of sufficient extent or by a momentary fault simulating current which is generated at the tripping of the GFCI to interrupt current across the reset terminals. If the load circuit possesses a true fault and passes test current from the reset circuit to ground that exceeds a predetermined level, and thus corroborates or confirms the cause of tripping as a hazardous fault condition, the reset circuit will remain open as long as the fault persists, and the GFCI will not be reset until the fault is cleared; however, if the predetermined level of current flow to ground does not exceed the predetermined amount, the reset circuit immediately closes, restores current flow across the reset terminals, and resets the GFCI to restore power to the load circuit. If the automatic reset circuit should fail closed, or become insensitive to fault for any reason and not respond to either a fault or the momentary simulated fault in opening the reset switching circuit, the continuous conductivity after tripping will keep the GFCI in the off-state. If the automatic reset circuit should fail open, the continuous interruption after tripping will also maintain the GFCI in the off-state, and in most embodiments would cause the GFCI to switch off simultaneously with the failure. These factors comprises a considerable fail-safe assurance of functioning in regard to the automatic reset circuit.